The use of activated carbon as a catalyst to promote the formation of glyphosate by oxidation of N-phosphonomethyliminodiacetic acid is described by Hershman, U.S. Pat. No. 3,969,398, where N-phosphonomethyliminodiacetic acid is prepared by reacting iminodiacetic acid with formaldehyde and phosphoric acid. Hershman reports teaching that glyphosate is produced by the oxidation of N-phosphonomethyliminodiacetic acid in the presence of activated carbon as a catalyst according to the following proposed reaction: EQU C.sub.5 H.sub.10 O.sub.7 NP+1/2O.sub.2 .fwdarw.C.sub.3 H.sub.8 O.sub.5 NP+CO.sub.2 +CH.sub.2 O
Rogers et al., U.S. Pat. No. 5,578,190 describes the use of carbon impregnated with various metals. The impregnated carbon is utilized to facilitate a hydrogenation reaction to produce glyphosate.
Cullen et al., International Publication Number WO 96/38455, describes the oxidation of N-phosphonomethyliminodiacetic acid with an oxidizing agent such as hydrogen peroxide in the presence of activated carbon as a catalyst. The oxidation reaction results in the production of glyphosate.
Chou, U.S. Pat. No. 4,624,937, describes the process for modifying an activated carbon capable of oxidizing tertiary amines and secondary amines in the presence of activated carbon. The process describes the modification of the activated carbon by an oxygen-containing gas and ammonia at temperatures from 800.degree. to 1200.degree. C. The modification process enhances the ability of the activated carbon to facilitate the oxidation of the tertiary or secondary amines.
All of the prior art for improving the production of glyphosate from N-phosphonomethyliminodiacetic acid has certain disadvantages, which make the process unattractive from a commercial standpoint. Chief among these is an inability to determine in a rapid and convenient manner the suitability of a char for such applications prior to its use, in particular the intrinsic catalytic activity of the char for glyphosate manufacture. As a result of this shortcoming, it is not possible to know or even to estimate during the preparation of a char the utility of the final product short of actual testing in the application itself.
Accordingly, it is the object of the present invention to provide an improved process for the manufacture of N-phosphonomethylglycine from N-phosphonomethyliminodiacetic acid in the presence of a gas containing molecular oxygen, such as pure oxygen, by contacting said acid with a low temperature catalytically active carbonaceous char in which the intrinsic catalytic activity of the char is measured and known prior to use. It is further the object of the present invention to estimate the intrinsic catalytic activity of the char using a test which is relatively cheap, quick, and simple in its execution and fairly indicative of the suitability of the char for the intended application.